Both natural and synthetic crystalline aluminosilicates are known and may generally be described as alumino-silicates of ordered internal structure having the following general formula: EQU M.sub.2/n 0:Al.sub.2 0.sub.3 : YSiO.sub.2 :ZH.sub.2 O
where M is a cation, n is its valence, Y the moles of silica, and Z the moles of the water of hydration.
When water of hydration is removed from the crystalline aluminosilicates, highly porous crystalline bodies are formed which contain extremely large adsorption areas inside each crystal. Cavities in the crystal structure lead to internal pores and form an interconnecting network of passages. The size of the pores is substantially constant, and this property has led to the use of crystalline aluminosilicates for the separation of materials according to molecular size or shape. For this reason, the crystalline aluminosilicates have sometimes been referred to as molecular sieves.
The crystalline structure of such molecular sieves consists basically of three-dimensional frameworks of SiO.sub.4 and AlO.sub.4 tetrahedra. Isomorphous substitution of boron or gallium for aluminum in a zeolite framework may be achieved. The tetrahedra are cross-linked by the sharing of oxygen atoms, and the electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, e.g., alkali metal or alkaline earth metal ions or other cationic metals and various combinations thereof. These cations are generally readily replaced by conventional ion-exchange techniques.
The spaces in the crystals between the tetrahedra ordinarily are occupied by water. When the crystals are treated to remove the water, the spaces remaining are available for adsorption of other molecules of a size and shape which permits their entry into the pores of the structure.
Molecular sieves have found application in a variety of processes which include ion exchange, selective adsorption and separation of compounds having different molecular dimensions such as hydrocarbon isomers, and the catalytic conversion of organic materials, especially catalytic cracking processes.
U.S. Pat. No. 3,123,441 discloses a lithium aluminum silicate zeolite having a lithium oxide to alumina ratio of 1:1 and a silica to alumina ratio of 2:1.
U.S. Pat. No. 3,411,874 discloses the preparation of a zeolite ZSM-2 which has the chemical formula M.sub.2/n 0:Al.sub.2 O.sub.3 :(3.3-4.0)SiO.sub.2 :ZH.sub.2 O. The composition includes lithium as the M specie and is known to have utility for selective adsorption and separation of compounds, such as hydrocarbon isomers. The zeolite is synthesized from a single mixture over a period of from three days up to three months.
In U.S. Pat. No. 3,415,736, lithium-containing crystalline aluminosilicate compositions are disclosed which are broadly recited to include (0.05-0.8)Li.sub.2 O:(0.95-0.2)Na.sub.2 O:Al.sub.2 O(2.0-6)SiO.sub.2 :(0-9)H.sub.2 O and, m specifically, (0.3-0.8)Li.sub.2 O:(0.7-0.2)Na.sub.2 O. Al.sub.2 O.sub.3 :(2.8-4)SiO.sub.2 : (0-9)H.sub.2 O. These zeolites are known as ZSM-3. They also are described as having utility in selective adsorptive separations, such as for hydrocarbon isomers. The crystalline ZSM-3 is recited to contain a hexagonal crystalline structure. The zeolite is typically synthesized from a combination of four solutions which form a gel from which the zeolite crystallizes over a period of hours or days.
In an article entitled, "Synthesis and Characterization of VPI-6" by Mark E. Davis, appearing in Molecular Sieves, (1992) pp 60-69, a crystalline zeolite having cubic and hexagonal intergrowth in the faujasite structure is disclosed. The synthesis of the zeolite involve aging a solution for 24 hours and indicates that aging is an important criteria of the synthesis. Specifically, the author of this article attempted to synthesize the zeolite in only the sodium cation form. The utility of the VPI-6 zeolite is recited to be as an adsorbent or ion exchange medium.
J. L. Lievens, et al. in an article "Cation Site Energies in Dehydrated Hexagonal Faujasite", appearing in ZEOLITES, 1992, vol. 12, July/August, pp 698-705, reviews properties of hexagonal faujasite designated as EMT. FAU/EMT intergrowths were also discerned in the studied EMT materials. Sodium was the cation which was involved in the cation site studies, and Si/Al ratios of 4.6 were specified.
U.S. Pat. No. 5,098,686 discloses faujasite compositions in which high Si/Al ratios are attempted, preferably above 3. Hexagonal and cubic structure mixtures are disclosed. All of the examples have compositions with Si/Al ratios above 3.7.
U.S. Pat. No. 5,116,590 discloses a zeolitic structure, ECR-35, which has a Si/Al ratio of 2:1 to 12:1, preferably 4. ECR-35 is an intergrowth of faujasite and Breck Structure Six (a nomenclature for hexagonal faujasite, subsequently EMT). Cation sites are occupied by tetraethylammonium and methyltriethylammonium cations.
J. A. Martens, et al. in an article entitled "Phase Discrimination with .sup.29 Si MAS MNR in EMT/FAU Zeolite Intergrowths", J. Phys. Chem. 1993, 97, pp 5132-5135, describes the evaluation of ZSM-2 and ZSM-3 in lithium exchanged format to determine the content and extent of any EMT and FAU phases in their crystal structures.
G. T. Kokotailo, et al., reported in "Synthesis and Structural Features of Zeolite ZSM-3", Molecular Sieve Zeolites--I, Amer. Chem, Soc., 1971, pp 109-121, the synthesis of ZSM-3 with a composition of (0.05-0.8)Li.sub.2 O:(0.2-0.95)Na.sub.2 O:Al.sub.2 O.sub.3 : (2-6)SiO.sub.2 :(0-9)H.sub.2 O.
D. E. W. Vaughan, et al., in "Synthesis and Characterization of Zeolite ZSM-20", in Zeolite Synthesis, Amer. Chem. Soc. 1989, pp 545-559, investigated the effect of potassium on the ZSM-20 material which was synthesized with a template cation and reported to have hexagonal and cubic crystal structure. As reported in Table 1, potassium had an adverse impact on the formation of the ZSM-20 structure.
The prior art fails to provide a composition that is both lithium cation rich and aluminum rich which produces a cubic/hexagonal intergrowth of FAU and EMT crystalline metallosilicate having significant adsorption utility, such as air separation. The present invention as set forth below uniquely achieves these goals to provide a high performance, novel, nitrogen-selective gas separation adsorbent.